Reduction of fuel consumption is one of the biggest technical themes in automobiles. As part of this, there have been increasingly strong demands made for the reduction of the weight of pneumatic tires.
The inner surface of a pneumatic tire typically has an inner liner comprising a low gas permeation rubber such as a butyl rubber or a halogenated butyl rubber so as to maintain a constant tire air pressure. A halogenated butyl rubber, however, has a large hysteresis loss, and therefore, when rippling occurs in the inner surface rubber of the carcass layer and the inner liner at the intervals between carcass cords after vulcanization of the tire, the inner liner rubber layer will deform along with the deformation of the carcass layer, and therefore, there will be the problem of an increase of the rolling resistance. Accordingly, in general, a rubber sheet called a tie rubber with a small hysteresis loss is interposed between the inner liner (typically halogenated butyl rubber) and inner surface rubber of the carcass layer to adhere the two. Therefore, in addition to the thickness of the inner liner of the halogenated butyl rubber, the thickness of the tie rubber is added and the thickness of the layers as a whole exceeds 1 mm (i.e., 1000 μm) which in the end causes an increase in the weight of the product.
Techniques have been proposed for using various materials, instead of a low gas permeation rubber such as a butyl rubber, as the inner liner of the pneumatic tire. For example, Japanese Examined Patent Publication (Kokoku) No. 47-31761 discloses coating the inner surface of a vulcanized tire with a solution or dispersion of synthetic resins such as polyvinylidene chloride, saturated polyester resin, polyamide resin having an air permeation coefficient (cm3 (standard state)/cm·sec·mmHg) of not more than 10×10−13 at 30° C. and not more than 50×10−13 at 70° C., at a thickness of 0.1 mm or less.
The technique disclosed in Japanese Examined Patent Publication (Kokoku) No. 47-31761 describes providing a covering layer of a synthetic resin having a specific air permeation coefficient on the inner circumferential surface of the carcass or the inner circumferential surface of the inner liner of the vulcanized tire and keeping the thickness of the synthetic resin covering layer to 0.1 mm or less, but the pneumatic tire described in this publication has problems in the adhesiveness of the rubber and synthetic resin. Further, there is the defect that the inner liner layer is inferior in moisture resistance (or water resistance).
Japanese Unexamined Patent Publication (Kokai) No. 5-330307 discloses halogenation of the inner surface of the tire (using a conventionally known chlorination solution, bromine solution, or iodine solution) and then forming a polymer film (thickness of 10 to 200 μm) of methoxymethylated nylon, copolymerized nylon, a blend of polyurethane and polyvinylidene chloride, or a blend of polyurethane and polyvinylidene fluoride over the inner surface.
Further, Japanese Unexamined Patent Publication (Kokai) No. 5-318618 discloses a pneumatic tire having a thin film of methoxymethylated nylon as an inner liner. According to this technique, the inner surface of the green tire is sprayed or coated with a solution or emulsion of methoxymethylated nylon and the tire then vulcanized or the inner surface of the tire after vulcanization is sprayed or coated with a solution or emulsion of methoxymethylated nylon to produce a pneumatic tire. In these publications as well, in addition to the defect of the poor water resistance of the thin films, there is the defect of a difficulty in maintaining uniformity of the film thickness.
Further, Japanese Unexamined Patent Publication (Kokai) No. 6-40207 has an example of use of a multilayer film having a low air permeation layer comprising a polyvinylidene chloride film or an ethylene-vinyl alcohol copolymer film and an adhesive layer composed of a polyolefin film, an aliphatic polyamide film, or a polyurethane film as the air permeation preventive layer of the tire. In this system, however, the low air permeation layer lacks flexibility and the film cannot track expansion or contraction of the material when the tire is in use, and thus the tire splits.
Further, Japanese Unexamined Patent Publication (Kokai) No. 5-508435 proposes the use, as a tire inner liner composition, of a composition comprised of a halogen-containing copolymer of C4 to C7 isomonoolefin and p-alkylstyrene containing carbon black, a plasticizer oil, and a vulcanization agent for the tire inner liner, but the inner liner has an insufficient air permeation coefficient and is not suitable for reducing the weight of the tire further.
Thus, the material for forming the gas barrier layer used for the inner liner of a pneumatic tire etc. is required to have flexibility and a gas barrier property, but no material has yet been presented which has both of these properties.
Generally speaking, a thermoplastic elastomer composition which is composed of a thermoplastic resin component as a continuous phase and an elastomer component as a dispersed phase and in which at least part of the elastomer component is cross-linked (vulcanized) has the rubber elasticity performance derived from the elastomer component which has generally been cross-linked in the past, and, due to the thermoplastic resin component forming the continuous phase, can be thermoplastically molded at a high temperature where it melts and becomes fluid. Consequently, a thermoplastic elastomer composition having this dispersed structure has the characteristic of enabling formation by processing techniques similar to those of plastics while maintaining the properties of a vulcanized rubber.
Therefore, a thermoplastic elastomer composition has the following basic advantages when compared with vulcanized rubber:
(1) a separate vulcanization process is not required.
(2) recycling of the products and the scrap produced during the processing are possible.
(3) lightening of the weight is possible.
Among these, in particular, a thermoplastic elastomer composition (where a part or all of the elastomer component forming the dispersed phase is cross-linked (vulcanized) during mixing with the thermoplastic resin which forms the continuous phase and is dynamically cross-linked (vulcanized)), can in particular give a product superior in the mechanical physical properties of a rubber elastomer, resistance to compression set, and resistance to oil and can be used, instead of conventional rubber, for auto parts, building materials, medical equipments, general industrial materials, etc.
Relating to this thermoplastic elastomer composition, the present inventors previously proposed and filed an application (Japanese Patent Application No. 7-150353) (i.e., Japanese Unexamined Patent Publication No. 8-259741), for a polymer composition for a tire having an air permeation coefficient of not more than 25×10−12 cc·cm/cm2 sec·cmHg and a Young's modulus of 1 to 500 MPa which is superior in the balance of the air permeation preventive property and the flexibility as a polymer composition for a tire, which can be used to reduce the weight of the tire, and which is composed of a blend of a specific amount of a thermoplastic resin having an air permeation coefficient of not more than 25×10−12 cc·cm/cm2 sec·cmHg and a Young's modulus of more than 500 MPa and a specific amount of an elastomer component having an air permeation coefficient of more than 25×1012 cc·cm/cm2·sec·cmHg and a Young's modulus of not more than 500 MPa.
This proposed composition had a superior function as a polymer composition for a tire, but when used as the inner liner or other air permeation preventive layer of a pneumatic tire had the problem that the adhesiveness with the rubber layer was not sufficient with the thermoplastic elastomer composition alone.
Further, the present inventors engaged in research to make the air permeation preventive property in the above polymer composition for a tire even more effective and proposed and filed an application (Japanese Patent Application No. 7-55929) (i.e., Japanese Unexamined Patent Publication No. 8-244402), for a thermoplastic resin composition for a tire forming an integral air permeation preventive layer by extruding a blend of at least two types of incompatible thermoplastic resins in the process of which one thermoplastic resin component among the thermoplastic resin components is not finally dispersed but is dispersed oriented in a flat form due to the shear stress at the time of extrusion as it is incompatible.
When this thermoplastic resin composition is used as the air permeation preventive layer of a pneumatic tire, since it is a thermoplastic resin composition, a sufficient air permeation preventive property can be obtained, but this was not sufficient to sufficiently control the flexibility and durability with respect to flexural fatigue and also there was the problem of an insufficient adhesiveness with the rubber layer with the thermoplastic resin composition alone.
Thus, as explained above, there is known a thermoplastic elastomer composition, having a thermoplastic resin component as a continuous phase and an elastomer component as a dispersed phase, which has rubber elasticity, is flexible, can be processed thermoplastically, and enables control of the air barrier property or gas barrier property. Further, there is known a thermoplastic resin composition which enables a layer having a gas permeation preventive property to be formed integrally inside the layers. A thermoplastic elastomer composition which has rubbery elasticity and can be thermoplastically processed and which enables the formation of a layer having bondability and other necessary functions integrally at the outside of the layers is not yet developed.
Other references of interest include: WO 2004/081107, WO 2004/081106, WO 2004/081108, WO 2004/081116, and WO 2004/081099.